The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to closed-loop control of wafer polishing in a chemical mechanical polishing system.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductor, semiconductor or insulator layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The effectiveness of a CMP process can be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the wafer surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the wafer and pad, and the force pressing the wafer against the pad.
A recurring problem in CMP is the “edge-effect,” in other words, the tendency of the wafer edge to be polished at a different rate than the wafer center. The edge effect typically results in non-uniform polishing at the wafer perimeter, for example, the outermost three to fifteen millimeters of a 200 millimeter (mm) wafer. A related problem is the “center slow effect,” in other words, the tendency of the center of the wafer to be underpolished.
Other factors also contribute to non-uniformity in the CMP process. For example, CMP processes are sensitive to differences among polishing pad from different lots, variations in batches of slurry, and process drifts that occur over time. In addition, CMP processes may vary with depending on environmental factors, such as temperature. The particular condition of the wafer and films deposited on the wafer also contribute to variations in the CMP process. Similarly, mechanical changes to the CMP system can affect the uniformity of the CMP process. Variations in the CMP process may occur slowly over time, for example, as a result of wear to the polishing pad. Other variations may occur as a result of a sudden change, such as when a new batch of slurry or a new polishing pad is used.
Using current techniques, it has been difficult to compensate for the foregoing variations in CMP processes to control wafer thickness dynamics. In particular, it has been difficult to control CMP processes to obtain a desired flatness or topography of the wafer surface. Similarly, it has been difficult to control CMP processes to obtain repeatable results for numerous wafers over a long period of time.